Vertical resistivity logging by measuring the electric field created by a time-varying magnetic field



Nov. 18. 1969 R. J. RUNGE 3,479,581

VERTICAL RESISTIVITY LOGGING BY MEASURING THE ELECTRIC FIELD CREATED BYA TIME-VARYING MAGNETIC FIELD Fled Aug. 24, 1967 3 Sheets-Sheet 1 SUPPLYAT1-0R EYs f" Nov. 18. 1969 R. J. RUNGE VERTICAL 'REsIsTIvITY Lo GGINGBY MEASURING THE ELECTRIC FIELD' CREATED BY A TIME-VARYING MAGNETICFIELD Filed Aug. 24, 1967 u voLTAG E 1 AMPLIFIER 3 Sheets-Sheet 2 lo vf6 INVENTOR RICHARD J. RUNGE Nov. 18. 1969 R. J. RUNGE 3,479,581

VERTICAL REsIsTIvITY LCGCING EY MEASUEING THE ELECTRIC FIELD CREATED EYA TIME-VAEYING MAGNETIC FIELD Filed Aug. 24, 196'? 3 Sheets-Sheet 3VERTICAL RESISTIVITY LOGGING BY MEASUR- ING THE ELECTRIC FIELD CREATEDBY A TIME-VARYING MAGNETIC FIELD Richard I. Runge, Anaheim, Calif.,assignorvto Chevron Research Company, San Francisco, Calif.,"`f acorporation of Delaware FiledfAug. 24, 1967, Ser. No. 662,981

Int. Cl. G01v 3/12 U.S. Cl. 324-6 v A'11 Claims ABsTkAcT oF THEDlscLosUiiE A vertical resistivity borehole logging system using anelongated current dipole to create a time-varying magnetic field inA.earth formations traversed byl the borehole that indpces a uniformelectric field varying in accordance with the vertical (across thebedding) conductivity or resistivity of the earth formations which ismeasured by :potential electrodes. As distingjiished from (1)conventiohal electrical logging (using lciw frequency or D.C.), orjl (2)induction logging (usirfg high frequency) bothl-,of which inherentlymeasure oily the horizontal resistivity perpendicular to the borehole,the present method measres vertical resistivity in the diectionparallelto the axis of the borehole.

This invention relates to the art of electric borehole logging an'd moreparticularly to a new method for determining the resistivity parallel tothe borehole in an anisotropic medium.

slt-is aprimary object of this invention to measurethffr'yrtical,resistivity of electrically anisotropic earth formait-ionssurrounding a well bore. Conventional electric and induction loggingtechniques are known to measure'dn such electrically anisotropicformat.. ons only res i vity in "a Vdirection perpendicular to 111e"borehole, a'i'ld generally along the horizontal bedding; his resistivityis called the horizontal resistivity. The phenomenon is commonly calledthe paradox of anisotropy (see Kunz and Moran,'Some Effects of FormationAnisotropy on Resistivity Measurements in Boreholes, Geophysics, vol.XXIII, No. '4 October 1958, pp. 770-794 at p.775). For isotropic mediathis paradox poses no problem and conventional techniques will suice todetermine resistivity. But due to* this paradox there has previqislybeen no Way to known the vertical resistivity in siich an anisotropicmedium and thus no means of determining the coefficient of anisotropy Ain an anisotropic medium, where in` which Rv=vertical resistivityRH=horizontal resistivity.

It will be evident that the anisotropy can readily be obtained once thevertical resistivity is measured because the horizontal resistivity iseasily measured by conventional techniques.

Recent developments in the art of electric logging and in particulardevelopments disclosed in Runge et al. U.S. Patent 3,256,480 concerningan ultra-long-spaced electric logging system (hereinafter referred to asthe ULSEL method) indicates the desirablility of knowing the lithologicparameter of anisotropy. For example, it has been `found that undercertain conditions of electrical anisotropy it is difcult to compute thedistance to a body of high electrical impedance without knowing thiscoeicient of anisotropy. As stated in lines 72-75 of column 6 in thespecification of this patent, the apparent distance 3,479,581 PatentedNov. 1 8, 19169 to a salt body (or A{other} body of lsignificantlydiiierent resistivity than the formations surrounding a well bore) in ahomogeneous, isotropic medium yis given by the formula .1

where p=the resistivity measured in the absence of a salt body ps=theresistivity measured in the presence of a salt body A=the spacingbetween the nearestv long spaced potential electrode and 'the currentelectrode in the ULSEL method 'i XAp=the apparent idistance to a sltbody.

The relationship between the apparent distance, XAp, givenin thisformula,'and the actual distance, X0, in an anisotropic medium is knownto be X Ap'=X0/)\, where tV/iscomposed of components diie to both themicroscopic anisotropy within stratigraphic layers and macroscopicanisotropy caused by successive stratigraphic layers with differentcondiictivities. Knowledge of the microscopic anisotropy aridamapping'of stratigraphic ,layers allows the composite @A to be c'omputed and accurate lateral distance measurements to bemade by theULSEL method. 1t is also possible to modify and improve the model Qonwhich tlie ULSEL method is based with a knowledge of the microscopicanisotropy.

If there is a dip 1in the beds near a body of high electrical impedance"then macroscopic anisotropy may obscure lthe data obtained under theULSEL method.v B y analyzing the efect'jof' dip on thieinterpretation ofdata obtained Aby applying, this method, it can be shown that forsediments `dippirig' at some angle away from the axis of a borehole, aknowledge of the composite anisotropy is useful in interpreting ULSELdata.

Knowledge of the1n anisotropy is also useful in differentiating betweensand beds, which are isotropic, and shale beds, which arenanisotropic. Amethod of determining the true value .of-l the coeliicient ofanisotropy, in effect, serves as a `sand-shale meter. Also, sand whichcontained shale as i,an impurity or which has been vinvaded by otherforeign substances has a coeflicient of anisotropy different unity. Byanalyzing and comparing data-it is possittl to estimate accurately theconstitution of beds of saiid;

It is therefore arimary object of this invention to measure electricalresistivity in a direction parallel to a well bore by circumventing theparadox `of anisotropy.

The general formula describing an electric field can be expressed as o tot where the first term on the right-hand side of the equation is thedivergence of the scalar potential and the second term -is the rate ofchange of the vector vpotential with time. For electric loggingpurposes'the rst term can be called the potential component of theelectric eld and may be due to the current which is injected into theformations surrounding a well bore by the current electrodes used inconventional electric logs. For electric logging purposes the secondterm can be calledfthe induced component of the electric field and inaccordance with thefmethod of this invention is induced in the for-.-mations surrounding a well bore by a time varying electromagnetic field;in conventional electric logs the second component is negligible becausethere is no significant time-varying magnetic field associated with thelogging sonde. It can be seen, then, that the potential field -Vqs isthe one that is utilized in conventional electric logs to measureresistivity. However, it is a strange fact of nature that this potentialfield cannot be used in conventional electric logging to vmeasurevertical resistivity. The potential field is directly dependent on thecurrent flow and it can be mathematically demonstrated that the currentdistribution associated with a conventional electric lo'g .'jshifts ifthe vertical resistivity is varied and that this shift is such that thepotential measured between two electrodes remains consxtant as thevertical resistivity is varied. Thus the paradox of anisotropy isgrounded on the insensitivity of the potential field to changes in thevertical (parallel to the bore hole) resistivity when measured along thevertical.

The present invention circumvents the paradox of anisotropy by either(1) rendering the potential field insignificant in comparison to theinduced field, or (2) eliminating the potential field altogether andcreating asignificant induced field; these two embodiments of theinvention will be denoted as the first and second embodiments,respectively,` C

Briefiy stated,i""the method of the present invention comprehendsestablishing a substantially uniform, timevarying electromagnetic fieldover ran elongated depth interval in earthformations surrounding a wellbore to induce a circularly uniform vertical electric field near thecenter of said depth interval, and then detecting the potentialdifference between a pair of electrodes spaced a given vertical,distance apart near the center of said depth interval asE an indicationof the resistivity of earth formations in aldirection parallel tb thewell bore and characteristic oft'the vertical resistivity of suchformations. j f

Further objects and advantages of the present invention will become.'apparent from thfe following detailed description of the method and itsapplication.

In the drawings; 3

FIGURE 1 is a schematic representation of a logging system illustratingthe first embodiment for practicing the method offthle presentinvention.

FIGURE 2 is `a side elevation view, partially in section of the upperhalf. of apparatus shown in FIGURE 1 for practicing the rst embodimentof this invention.

FIGURE 2A 'is a view, partially in section, of the lower end of the`apparatus shown inl FIGURE 1.

FIGURE 2B;{is a cross-sectional view of the lower end of the apparatusof FIGURE 1 modified to the nonconductive=-fbrm of the secondembodiment.

FIGURE 3 t"schematically illustrates the magnetic field created by acurrent oscillating in a dipole antenna and the corresponding inducedelectric field, useful in a theoretical explanation of the method of theinvention.

FIGURE 4 ris fa side elevation view partially in crosssection of analternative form, or second embodiment, of apparatus suitableforpracticing the method of the present invention", wherein the upperhalf of the dipole antenna is wrapped around the logging cable.

FIGURE 4A is a cross-sectional view of the bridle and the upper portionof the dipole antenna of the apparatus shown inflFIGURE 4.

FIGURES 4B and 4C represent the upper and lower ends, respectively", ofthe apparatus of FIGURE 4 when the method of the invention is practicedusing the first embodiment. I

FIGURE 5 is-,a cross-sectional view of an alternative form of apparatusfor practicing the second embodiment wherein the logging sonde is asingle unit.

In both embodiments of the present invention a dipole antenna systemfrom 30 to 100 feet long is utilized to produce a time varying magneticfield which induces a vertical" electrical field in the formationssurrounding the well bore. Since the current oscillating in the dipoleantenna is vertical, the induced electric field is also vertical in theformations. Once this vertical induced fieldl dominates or replaces thepotential field a measurement of the potential between two potentialelectrodes will measure directly andsimply the vertical resistivity ofthe surrounding formations.

The dipole antenna can have several forms such as thin wire10 in FIGURE2, cylindrical rod 32 in FIG- URES 4 and 4A, or cylindrical rod 14 inFIGURE 5. The dipole antenna can be driven directly as shown in FIGURE-2by a direct tie-in of oscillafor 36 to thin wire antenna 10; or thedipole antenna `can be driven by inductive coupling as shown by the pairof coupled coils 40 in FIGURES 4 and 5. In order Athat the maximumcurrent ofscillates in the antenna it is desirablev that the coils bematched so that a resonance ,circuit is obtained.

In a fist embodiment of the present invention the induced field is madeto dominate the potential field by fixing the parameters of a sonde so;`that the ratio of the absolute value of the induced field to theabsolute value of;y the potential field is made greater than 20 in theregion in which the measuring electrodes are placed. This firstlembodiment of the invention is illustrated by FIGURE, 1, FIGURES 2 and2A, and by FIGURES 4B and 4Cfiwhen viewed in conjunction with FIGURE 4.As is readily apparent the primary' physical difference between thesecond embodiment and the first embodiment is that the ends of thedipole antenna are insulated in the second embodiment, as illustrated byfiberglass insulation 16 in FIGURE 2B, insulated cylindrical rod 14 inFIGURE 5, insulated cylindrical rod 32 in FIG- URE 4 tand by insulatedcylindrical sheath 12 in FIG- URE 4g'. On the other hand, theends of thedipole antenna are in electrical contact with the surrounding formationsin the first embodiment, as illustrated by spherical current electrode48 in FIGURES 1 and 2A,

conducting head 54 in FIGURES 1 and 2, exposed upper end 50 ofcylindrical shell 72 in FIGURES 4A and 4B, and lower current electrode52 in FIGURE 4C. In these embodiments, current s"flows into and isreceived from the surrounding formations, as illustrated by currentlines 92, in FIGURE 1".: The physics of this difference is great becausethere is yirtually no potential field at 'the ends of the dipole in thesecond embodiment, and the'vertical resistivity can be nieasured withoutconsideringf'any effect due to the potential field. However, due to theformation current flow-from the ends of the dipole the first embodimentit is possible to drive a somewhat larger current in theu'dipole antennathan is possible in the second embodiment; thus, the 'induced fieldoffthe first embodiment is 'stronger than the induced field of thesecond embodiment.

It cany be mathematically demonstrated that for the first embodiment theratio of the absolute value of the induced field to :the absolute valueof the potential field in a limited region called the near central zoneis given by The'near central zone is defined to be the region in whichthe distance from the dipole antenna as well as the distance from animaginary line bisecting the dipole antenna are both much less thanone-half the total length of the dipole antenna. Potential measuringelectrodes '20 and 22 in FIGURES l and j52vand band electrodes 24 and 26in FIGURE 4 are located in this near central zone.

For the rst embodiment it can be seen from the above ratio formula thatthe Htwo parameters that can readily be varied are the frequency f andthe dipole length L. The ratio is highly responsive to changes in lengthbecause of the exponential relationship. Also, even though it would seemthat the ratio cduld be enlarged by simply using a high frequency it isdsirable to keep the frequency in the range of 100 kHz. to'llOQO kHz.because the skin depth, ).0, is inversely proportional to the squareroot of the frequency: the higher the frequency, the shorter the skindepth. And, as can seen from the ratio formula, the

ratio is proportionalio ,the skin depth in a complicated way; the resultis tha'tthe skin depth should not be made too small. The chartgpelowshows the dependence of the ratio on the length at la point one-tenth ofa meter from the axis of the antenna on a line bisecting the antenna ifthe frequency is set at 100 kHz. and if the medium has a verticalresistivity of 10 ohm-meters.

For these particular conditions it can be seen that the sonde should notbe less than 36 feet long if a ratio of to 1 is to obtain. 1-

The induced field that exists in both embodiments and which is shown inFIGURE 3 is vertical in the formations. As a result of this verticalproperty it can be shown that, for both embodiments, the verticalresistivity pv is given by' the formula:

:exp a) pv IEZ |=Absolute Value of the Vertical Component of theElectric Field Amplitude=Absolute Value of the Measured ElectromotiveForce divided by the Vertical Distance between the Measuring Electrodesa, =Constants where In both embodiments of the present invention powersource 60, shown in FIGURE 1, supplies power vla power line 58 shown inFIGURES 2, 4, 4A and 5, to an oscillator, shown as 36 in FIGURES 2 and3, and as 34 in FIGURES 4 and 5, line 58 also supplies power to voltageamplifier 38, shown in FIGURES 2, 3, 4 and 5. The

voltage measured by the potential electrodes and amplied by voltageamplifier 38 is transmitted via line 56, of cable 42, to a recorder,such as pen galvanometer 46, shown in FIGURE l, to trace a record of thevoltage on visual recording means, chart 44. Depth generator 74indicates on chart 44 the depth at which the voltage was measured.Voltage amplifier 38 and oscillators 34 and 36 are constructed with aminimum of electromagnetically metallic parts to minimize anyinterference with the time varying magnetic field generated by thedipoleantenna. Also, the voltage signal sent from voltage amplifier 38via line 56 may be pulsed on a shared time basis with or altered infrequency from the current oscillating in the antenna to` minimizepickup 4,by line 56 from the oscillating magnetic field. Further, sincethe measured potential line 56 may pick up interference signals l:frompower line 58 if an alternating power'l source is use1d,`it is desirablefor power supply 60 to be a source of .direct current. y

Cable 42 is a standard strength logging "cable, used Ato move sonde 17in the borehole; it isfdrivfen by a standard cable winch and motorcombination designated schematically as 76 in FIGURE 1 Cable 42 isconnected to sonde 17 by a standard cable head 90.

Sonde 17, housing the dipole antenna, may be constructed of any suitablenon-magnetic and non-conducting material. In FIGURES 1 and 2 the housingis shown to be'formed as molded rod 16 and molded cylindrical casing 18.In FIGURE 5, it is shown to be cylindrical housing 70, and in FIGURE 4as cylindrical housing ,68. A` special advantage of the-form ofapparatus shown in FIGURE 4 is that only the lower half of sonde 68 hasto be constructed as a special housing: The upperlialf of the dipoleantenna is formed s cylindrical sllell 12 insulated from cable 42 andfrom the borehole by rubber boot72.

The upper and lower halves of the antenna are connected by insulatedwire 92 running through cable head coupling 90.

In running the sonde of FIGURES 1 and 2 into the borehole, deformablepads 62 and 64 are held out of contact with the walls of the well boreandthe surrounding earth formations. For this purpose catch 84 onsolenoid 66 holds deformable pads 62 ,and 64 away from the sides bycompressing steel springs 82 and 86 by holding collar 8S in the phantomposition of FIGURE 2. When the sonde is at the lowermost point in theborehole, switch 78 on the surface activates solenoid 66 via line8,0.causing catch 84 to retract and release collar 85 so that springs82l and 86 force deformable pads'62 and 64 against the formations tobring contacts 20 and 22 againstthe side wall of the bore hole. Such aposition may be required' if a highly resistive drilling fluid, such asan oil base or emulsion mud, is present in the well bore.

It is a further object of the present invention to operate as a newmethod of obtaining thin bed,v resolutions. In such operations,potential measuring electrodes 20 and 22, shown in FIGURE 1, or bandelectrodes 24 and 26; shown in FIGURE 4, and band electrodes 28 and 30,shownv in FIGURE 5, can be less than two feet apart. Consequently, it ispossible to detect with great accuracy the depth in the bore hole atwhich there is a shift in vertical resistivity to detect and interfacesuch as 88, shown in FIGURES 1, 2 and 3. Since the paradox ofanisotropytis circumvented by the method of the present invention, th'potential measured by the electrodes in such an example is affected onlyby the vertical resistivity of the formations with which it is in directphysical contact. Thus anomalies at a lateral distance from the edgeoft-the well bore do not hamper the near. instantaneous detection ofinterfaces between beds with varying vertical resistivities. Animportant use,'then, of this invention is to map thin bed stratigraphywhere commercial accumulations of oil may be present.

It might be suspected that the properties of the drilling fluid intheborehole would have some effect on the measurement as described. Inparticular, it might be suspected that a drilling fluid of very lowresistivitywould have the effect of shorting out the potentialmeasurement. However, a mathematical analysis, involving vMaxwellsequations, taking into account expected operating c ondtions ofr thetool, typical mud resistivities, hole diameters, and formationresistivities, indicates that the effect of the hole is negligible overa considerable range of these parameters. By this is meant theelectrical parameters, in particular, the conductivity of the mud, whichis not expected to have values outside the range of 1 to 10 mhos permeter.

' It is evident that other forms of apparatus may be used to practicethe method of the present invention. Indeed, it

is likely that individual aspects of the several forms illustrated inthe drawings could be combined to produce new forms. In essence,however, they all would rely on the concept of circumventing the paradoxof anisotropy by rendering the potential field insignificant incomparison to an induced electric field or by eliminating the potentialfield, and using only an induced electric field. 'Ihe verticalresistivity is then measured as a simple function of the electromotiveforce between two potential electrodes.

All such modifications coming within the scope of the following claimsare intended to be included therein.

I claim: "t

1. In electric logging of an earth formation exhibiting differentelectrical resistivities parallel and transverse to the bedding plane ofsaid earth formation, a method of determining the resistivity of saidearth formations parallel to a well bore traversing said earthformations over a known depth interval which comprises generating withinsaid well borea time-varying magnetic field extending verticallysubstantially above and below said known depth interval so that oversaid known depth interval said magneticfield forms circular lines offorce surrounding said well bore, with the planes of said circles beingsubstantially perpendicular to the axis of said well bore and thestrength of said field being substantially uniform in the verticaldirection over said depth interval, measuring an electricalcharacteristic of the electric field induced by said magnetic fieldbetween a pair of electrodes spaced a-fixed vertical distance apartabout the center of said electromagnetic field and within said knowndepth interval, and indicating said electrical characteristic over saidfixed 4vertical distance as a measure of the resistivity of said earthformations parallel to said well bore.

2. A method of determining the resistivity of an earth formationparallel to a well bore over a known depth interval which comprisesgenerating in said earth formation an elongated alternating currentelectromagnetic field extending a substantial distance above and belowsaid known depth interval, so that the lines of force of saidelectromagnetic field over said known depth interval are substantiallyparallel to said well bore over said known depth interval, measuring anelectrical characteristic of the induced electric field between a pairof electrodes spaced a fixed vertical distance apart near the center ofsaid electromagnetic field and spanning said known depth interval, saidelectrodes being in electrical contact with said earth formation, andrecording said electrical characteristic oversaid known depth intervalin accordance with the depth of said electrodes in said well bore as anindication of the resistivity of said earth formation parallel to saidwell bore.

3. A method of determining the cross-bedding resistivity of earthformations traversed lby a well bore which comprises traversing saidwell bore with an elongated antenna, driving said antenna at a frequencyin the range of from about 100 kHz. to 1000 kHz. to establish analternating current electrical field oriented substantially parallel toand over a substantial lengh along the axis of said well bore traversingsaid earth formations, and measuring between two points spanning a fixeddistance short relative to the length of said electrical field andsubstantially syminetrical about the midpoint of said antenna anelectrical characteristic of the electromotive force `induced in said Iformations as an indication of the crossbedding resistivity of saidearth formations traversed by said wellbore;

4. The method` of claim 3 wherein said measured elec tricalcharacteristic is resistivity and said resistivity is recorded inaccordance with the depth of said antenna in said well bore;v with theadditional steps of measuring the in-bedding resistivity of saidformations and then indicating the true' anisotropy of said formationsin accordance with the measured cross-bedding and in-beddingresistivities over the same depth interval of said well bore from theformula: where `=anisotropy Rv=vertical resistivity RH=horizontalresistivity 5. A method in accordance with claim 3 wherein the ends ofsaid elongated antenna are in electrical contact with said earthformations to pass current into and collect current from said earthformations during each reversal of current flow.

6. A method in accordance wilh claim 3 wherein the ends of saidelongated antenna are insulated to prevent electrical current flow fromsaid ends into and out of said earth formations.

7. A method of determining the vertical resistivity of earth formationsat a known depth in, and immediately surrounding, a well bore whichmethod comprises:

(a) traversing a known interval in said well bore with a dipole antennahaving a length L,

(b) electrically driving said antenna at a frequency f so that theabsolute value; of the ratio of the induced electric field to thevertical component of the potential field is greater than 20 in the nearcentral zone of said antenna system, said near central zone fbeingwithin the distances from the center of said antenna system and the axisof said antenna system are both much less than L72, said absolute valuebeing given by the formula:

@l al@ exp (L/2x0) 1n (Vl) E J5 r where 1r=3.1416 a0=the permeability offree space =41r l0'I henries/meter f=the frequency in Hertz a0=theconductivity of 'said earth formations in mbo/meter L=the length of saiddipole antenna system meters r=the distance away .from the axis of saidantenna system meters 7\0=the skin depth=5\/m meters,

(c) measuring the electromotive force between a pair of potentialelectrodes spaced a fixed vertical distance apart in said near crfitralzone, and

(d) recording the vertical resistivity pv of said earth formationsspanned by saiid4 pir of electrodes as a function of said measuredelectromotive force in accordance with the formla:

*l p :exp (im where EZ=Absolute Value of the vertical component of theElectric Field Amplitude=Absolute Value of said Measured'ElectromotiveForce divided by said Fixed Vertical Distance a,`=Constants 8. A methodof measuring over a known depth interval the true vertical resistivityof an earth formation traversed by a well bore whichfc'omprises inducinga uniform time-varying electromagnetic field extending sufficientlyaboye and lbelow said knbwn interval of said formation to createcircularly uniform lines of force that are substantially parallel to theaxis of said well bore above and below said knowninterval, and thenwithin said known interval, detecting with a pair of vertically spacedelectrodes spanning said known interval an electrical characteristic ofthe resultant induced vertical electric field as a measure of the truevertical resistivity of said earth formation. -f

9. Apparatus for measuring the conductivity orresistivity of an earthformation parallel to the axis of a well bore penetrating said formationwhich comprises r:

(a) an elongated conductor element adapted to be positioned parallel toand over an extended length along the axis of said fborehole,

(b) a source of alternating current having a frequency, in the range offrom about 100 kHz. to'"'1000 kHz.,

l(c) lneans for connecting said source to said conductor element togenerate a symmetrical electromagnetic eld concentric with and uniformlyperpendicular to the central portion of said conductor element,

(d) a pair of electrodes symmetrically positioned:- with respect to themid-point of said conductor elrnent and electrically insulatedtherefrom, said electrodes spanning only said central portion of saidconductor element wherein the resultant electric field is' substantiallyparallel to said conductor element,

(e).,means for connecting potential measuring means to said electrodesto indicate the electrical potntial across said pair of electrodes whensaid sourceA and said conductor element are applying a time-varyingelectromagnetic eld to earth formations,

(f) means for traversing said conductor element and said pair ofelectrodes through said well bore, and

(g) means for recording the output of said potential measuring means inaccordance with the depth of said electrodes in said Well bore toindicate the conductivity or resistivity of earth formations parallel tothe axis of said-fwell fbore. 10. Apparatus in accordance with claim 9in which said source of alternating current is an oscillator and saidmeans for connecting said source to said conductor element is aninductive coupling.

11. Apparatus in accordance with claim 10 in which said inductivecoupling a resonance circuit at the fre quency of said oscillatorlReferences Cited UNITED SIATES PATENTS 2,018,080 10/ 1935 Martienssen324-5 2,159,418 5/1939 Babcock 324-1 2,169,685 8/1939 Evten 324-12,222,182 11/ 1940 Mounce et al 324-5 XR 2,404,622 7/ 1946 Doan 324-102,415,364 2/ 1947 Mounce 324-1 2,951,982 9/ 1960 sfhuster 324-63,012,189 12/1961 D oll 324--3 XR 3,087,111 4/1963 Lhan et a1. 324-13,124,742 3/ 1964 Schneider 324-1 3,305,771 2/1967 Aips 324--6 GERARD R.STRECKER, Primary Examiner U.S. C1. X.R.

